2) Some of the album art is missing, low-resolution. How do I ensure that the best resolution album art is uploaded to my iTunes match database & also subsequently replaced on my existing local copies on my desktop iTunes app?

iTunes Match does not use Metadata to match your music. From what I can tell, it uses algorithms that detect the song to search for (for example from the waveform, bpm, track time etc). I know this because I tested wrongly tagged items which matched it correctly.

iTunes Match will use the meta data that YOU provide, and you will see the same meta data on all devices, including artwork.

A few other points to consider:

- ITM has a 25k song limit, so the size of your library in GB is irrelevant.
- When matches a song, it is available in the cloud in 256k AAC file, which can be downloaded on any other device.
- The original file on your Mac stays in iTunes, unless you delete it and re-download from the cloud (which is great if you had a lower bitrate mp3 to start with).
- If the song is not matched, then it will be uploaded to the cloud and available to download.
- Anything higher than 256k will be converted to 256 AAC. Unmatched songs cannot be re-matched. Ever.